Thermal neutron detectors usually employ materials with 10B (boron with 10 nucleons, i.e. 5 protons and 5 neutrons) or 3He (2 protons and 1 neutron). 157Gd, 6Li and a few other isotopes are also sometimes used but methods for incorporating them in large volume detectors have not been developed with the exception of some 6Li-based efforts.
Natural boron is approximately 20% 10B and 80% 11B. The 10B-based detectors are more common because almost all 3He comes from reprocessing nuclear waste, 3He is in high demand, and 3He is consequently very expensive. Most 10B-based detectors utilize BF3 and are typically a few cm in diameter with the BF3 at typically from one half to three atmosphere pressure. BF3 is toxic and must be carefully contained. For 10B, 3He and 6Li-based detectors, most employ systems to detect the electronic pulses or light coming from the ionization produced by the resultant decay products as the ions slow down in surrounding media. A variety of ionization chambers, multi wire proportional chambers (MWPC), gas electron multiplier (GEM), straw tube, solar blind photomultipliers, solid state photomultipliers, linear strip sensors, etc. are used. Typical sizes for BF3-based thermal neutron detectors are several cm in diameter and length and with associated high voltages in the range of 1,500-2,000 volts. Sizes of 3He-based thermal neutron detectors range from a few cm in most dimensions to ones for scientific research that may approach a meter in area with a several cm in thickness. 6Li-based detectors typically disperse 6Li in various plastic scintillator materials. To achieve adequate sensitivity, 3He-based detectors frequently require operation at pressures of several atmospheres, the addition of other gases such as propane and CF4, and a range of high voltages.
3He has a large cross section of 5,330 barns for the absorption of thermal neutrons and the reaction proceeds as:n+3He→p(0.573 MeV)+3H(0.191 MeV)While 3He has certain advantage in some implementations for achieving relatively high spatial resolution, 3He-based detection has limitations due to its limitations for making large, lightweight, and efficient thermal neutron detectors that can operate well at atmospheric pressure as well as at pressures from 0.001 atmosphere to over 5 atmospheres.
The primary limitation for 6Li-based detectors is that they typically require a solid or liquid scintillation material that results in unwanted background signals from other ionizing particles that may be present in the environment. In addition, the 6Li cross section for absorption of thermal neutrons is less than the 10B cross section for absorption of thermal neutrons.
10B has a large cross section of 3,835 barns for the absorption of thermal neutrons that can be exploited for the detection of the presence of thermal neutrons. The thermal neutron absorption reaction proceeds as:94%: n+10B→11B*→4He(1.47 MeV)+7Li(0.84 MeV)+gamma(0.48 MeV)6%: n+10B→11B*→4He(1.78 MeV)+7Li(1.02 MeV)The 11B* state lasts about 1E-12 seconds. The gamma, when present, comes from the decay of an excited state of 7Li.
Following absorption of the neutron the 4He and 7Li lose their kinetic energy by ionization loss in the surrounding material and the 0.48 MeV gamma, when present, is absorbed by the surrounding material. The occurrence of the neutron absorption on the 10B can be inferred by detecting the ionization losses of the 4He and 7Li ions or for 94% of the decays or by detecting the 0.48 MeV gamma when present. Some systems do both. For example, in some media the ionization losses produce light that can be detected by photon detectors such as photomultiplier tubes, solar blind photomultipliers, silicon photomultiplier (SiPM) arrays, large area avalanche photodiodes (LAAPD), etc. MWPCs, GEMs, straw tube and linear strip detectors that collect the ion pairs created in the surrounding media can also be used
Position and time sensitive fast neutron detectors often employ scattering (also known as recoil) methods where the fast neutrons scatter from light nuclei, such as protons or helium (4He), to produce the respective recoiling protons or helium ions that then ionize the surrounding materials. The ionization energy is then detected by scintillation or proportional counters. Issues with this methodology include relatively low efficiency and background noise from the inclusion of relatively low energy, i.e. slow, neutrons and other particles in the signal. Thermalizing fast neutron detectors infer the existence of fast neutrons by first slowing the fast neutrons in hydrogen-rich moderators and then detecting the thermal neutrons. All of these methods also have issues with eliminating gamma ray backgrounds through a variety of techniques to include pulse shape discrimination. In addition, the thermalizing methods also spread the signal that can be much less than a microsecond to time periods of many tens to hundreds of microseconds. In addition, methods that rely on producing thermal neutrons for fast neutron detection have backgrounds from the presence of other thermal neutrons that are typically present. Fast neutron fission chambers are available that typically use proportional counter technology. They have good rejection of gamma rays and when made with 238U as primarily sensitive to fast neutrons. The neutron fission chambers may have good timing resolution, but typically are limited in spatial resolution and total cross section.